The invention describes a Distributed Bragg Reflector and method for making utilizing multiple layers of AlGaN/GaN and, more particularly, utilizing an interlayer to achieve sufficient reflectivity for a vertical cavity laser.
The growth of epitaxial distributed Bragg reflectors (DBRs) with high crystalline quality is important in the successful development of infrared and red vertical-cavity surface-emitting lasers (VCSELs). The development of III-Nitride-based VCSELs for short-wavelength (visible and ultraviolet) applications requires the preparation of crack-free, highly reflective (that is, high R) (Al,Ga)N/GaN DBRs. Difficulties arise in the preparation of crack-free highly reflective (Al,Ga)N/GaN DBRs owing to the large lattice mismatch between GaN and AlN (approximately 2.4%).
Because multiple passes of optical waves are required in VCSELs due to a short cavity (gain) length, highly reflective mirrors (typically R greater than 99%) are required for low threshold operation. The low contrast in index of refraction between AlN and GaN (and thus the ternary alloys) necessitates the use of a large number of pairs of mirrors to achieve such reflectivities. Someya and Arakawa (Someya, T., and Arakawa, Y., Appl. Phys. Lett., 1998, 73, 3653-3655) reported the crack-free growth of a 35-pair Al0.34Ga0.66N/GaN DBR with reflectivity up to 96% at 390 nm. It was emphasized in that work that the thickness of the (high temperature-grown) GaN layer must be restricted to 0.4 xcexcm or less to avoid sample cracking. Langer et al. (Langer, R., Barski, A., Simon, J., Pelekanos, N. T., Konovalov, O., Andre, R., and Dang, L. S., Appl. Phys. Lett., 1999, 74, 3610-3612) reported a maximum reflectivity of 93% at 473 nm with 30 pairs of Al0.4lGa0.59N/GaN DBRs. Krestnikov et al. (Krestnikov, I., Lundin, W., Sakharov, A., Semenov, V., Usikov, A., Tsatsul""nikov, A., Alferov, Zh., Ledentsov, N., Hoffman, A., and Bimberg, D., Appl. Phys. Lett., 1999, 75, 1192-1194) employed a 1.1 xcexcm Al0.08Ga0.92N template on sapphire for stress compensation and showed a reflectivity of 96% at 401 nm with 37 pairs of Al0.15Ga0.085N/GaN DBR mirrors. More recently, Ng et al. (Ng. H., Moustakas, T., and Chu, S., Appl. Phys. Lett., 2000, 76, 2818-2820) explored DBR mirrors consisting of binary AlN and GaN for increased contrast in the index of refraction. A 99% reflectivity at 467 nm was obtained with one specific structure that employed approximately 20 to 25 pairs of DBR mirrors. A network of cracks was observed which was attributed to the large tensile stress between the two binary compounds. Kim (U.S. Pat. No. 6,306,672, issued on Oct. 23, 2001) describes the formation of DBRs for use in a VCSEL diode where low refractive index air layers are incorporated into the DBR to achieve desired reflectivity. Etching is used to form air layers out of conventional AlGaN or GaN material. These DBRs are utilized in forming a VCSEL diode. Waldrip et al. (Waldrip, K. E., Han, J., Figiel, J. J., Zhou, H., Makarona, E. and Nurmikko, A. V., Appl. Physics Letters, 2001, 78, 22, 3205-3207; incorporated herein by reference) describe the use of interlayers in DBR structures to address the mismatch of the AlGaN/GaN layers and achieve high reflectivity DBR structures without cracks.